Well a bowl floats as it displaces more water than it weighs. A real world bowl might weigh 1 lb but displace enough water to hold 5 without flooding the bowl.
The thing is things have a different ratio between their weight and how much they float. For example a log floats around 50% its volume so a 500 lb log could carry 500 lbs of non log.
Hollow that log out and you can not lose capacity. A dug out canoe might weigh 100 lbs so it should carry now 900 lbs (500 + 400)
There isn't a simple ratio and right now its a one ratio for all boats.
Towing floatables, like trunks and logs, is a separate issue.